The present invention is related to a packed plug for a fluid valve and more particularly to a novel means for maintaining a tight shut-off for the valve with a low stem force.
In the proper operation of a fluid control valve, it is highly advantageous for the valve structure to include a means to provide a tight valve shut-off without requiring high loading forces on the stem. It is evident that a tight valve shut-off is desireable for an effective valve action. When the closed condition of the valve is not functional to stop all fluid flow, the valve is inadequate in one of its primary objectives, that is to stop fluid flow through a system. Yet, for a convenient and easy valve action, it is desireable to maintain a tight valve shut-off with a minimum loading force on the valve stem. Consequently, the prior art has proposed several valve structures for maintaining leak-tight sealing between the valve plug and the valve body, without a high stem force. For example, an "O"-ring seal has been mounted outside of the plug to prevent leakage between the plug and the valve body. A disadvantage of this approach, however, is that the "O" ring is effective only at low temperature operations. In other prior art devices, piston rings have been utilized as sealing elements. However, these valves have proven to be limited in their versatility inasmuch as the piston rings are inadequate to prevent leakage in some applications. In still other prior art devices, the valve includes a double-seated plug which has been found to be unsatisfactory due to the fact that the valve stem must maintain a seating force on two valve seats simultaneously.
It is a primary objective of the present invention to provide a valve capable of a leak-tight shut-off with a minimum force on the valve stem and which at the same time overcomes the disadvantage of the above-discussed prior art devices. Generally, the valve of the present invention comprises a valve body including an inlet passage, an outlet passage and an internal web portion. Mounted within the web portion and in a sealed relation to the walls of the valve body is a cylindrical member provided with large fluid passageways whereby fluid flow through the valve enters through the input passage and flows through the fluid passageways of the cylindrical member to the outlet passage. One of the passageways is provided with a tapered annular valve seat. A valve plug is mounted for controlled axial movement within the confines of the cylindrical member. The lowermost end of the valve plug is provided with a tapered annular valve surface which is arranged for cooperation with the valve seat to close the valve.
In accordance with the invention, the valve plug comprises an upper plug member and a lower plug member, each fastened to a valve stem arranged to impart controlled axial movement to the two-member plug. The two plug members are mounted in an axially spaced relation to each other and a packing element is held in a snug fit between the members to provide a sealing element for the valve plug at the interface thereof with the interior wall surface of the cylindrical member. The lower valve plug member is of a generally inverted cup-shaped configuration including the tapered annular valve surface of the plug at the open lowermost end thereof. The valve stem extends within the confines of the cup-shaped, lower valve plug member, and an axially resilient element acts between the end of the valve stem and the lower valve plug member to urge the lower valve plug member toward the upper valve plug member, thereby maintaining the packing element in the snug-fitting mount between the plug members.
To advantage, passage-forming means are provided in each of the valve plug members to permit a leakage of fluid above the valve plug. This will allow balancing of the fluid pressure above and below the valve plug to facilitate operation of the valve. In a specific embodiment of the present invention, a slight area unbalance is provided between the upper and lower surfaces of the valve plug whereby the inlet pressure is continuously acting to force the lower valve plug member upwardly against the stem thereby assisting the axially resilient member to maintain the snug fit of the packing element between the upper and lower valve plug members.
In the operation of the valve, the axially resilient member always acts to generate a light packing load on the seal. However, when the valve plug is in its lowermost position, the annular valve surface will mate with the valve seat to close the valve, and the axial force exerted by the valve stem will be transmitted directly through the upper valve plug member and packing element to the lower valve plug member. The stem force will provide the necessary compression for a leak-tight sealing by the packing element as well as the main contact load necessary for a tight shut-off at the valve seat.
For a more complete understanding of these and other features and advantages of the present invention, reference should be made to the following detailed description of preferred embodiments of the invention and to the accompanying drawings.